Surface Roughness and Micropitting -day1_sessioni_4_rem_winkelmann.pdf

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Surface Roughness and Micropitting

Lane Winkelmann

Director of Services

National Renewable Energy LaboratoryWind Turbine Tribology Seminar

November 15-17, 2011



•

Introduction• Micropitting Basics

• Solution

– Superfinishing

• How it Works

• Why it Works

• Test Data

• History

• Conclusions

• Questions

Agenda



• Wind turbines have grown larger (1.5-6 MW)

– Led to large, multistage gearboxes

– Gearbox reliability has suffered

– Many require replacement/overhaul at 5-7 years – Drives up the cost of ownership

Introduction

(Source: NREL)



• Large turbine blades create massive torque

loads on the drivetrain

• Gears operate at relatively low speeds

• Operate in variable conditions – Temperature fluctuations

– Standstill

– Multiple start/stop

– Idling

• Operate under variable loads

What are the issues?



• Lubricants are a compromise for gears, bearingsand seals

– Thick enough for gears, thin enough for bearings

– Boundary/Mixed lubrication conditions

– Contact fatigue

– Plastic deformation of metal asperities

– Particles in lubricant

• Filtration can be a challenge – FOD

Current Lubrication Systems



• Corrosion

• Abrasion

• FOD Damage

• Micropitting

Typical Results After Operation



Micropitting Basics



Micropitting Basics-Tooth Mesh Sliding

• Driver

– Initial contact near root

– Rolls up

– Sliding is away from pitchline

• Driven

– Initial contact near tip

– Rolls down

– Sliding is towards pitchline

• Cracks

– Grow opposite direction of sliding

– Converge at pitchline• Hydraulic Pressure Propagation

– Negative sliding at dedenda accelerates crack

propagation by the hydraulic-pressure-propagation

mechanism proposed by Stewart Way.

(Source: Robert Errichello)



Micropitting-Surface Topography

• Micropitting attacks high points

– Crests of undulations

– Peaks of scallops

– Ridges of grinding lay – Edges of grinding scratches

– Shoulders of debris damage

• Micropitting locations

– Can be found along high points

– Asperities – Typically located in the dedendum

– Appears as a “Grey Stain”

(Source: Robert Errichello)



• Major replacement cause of highly

loaded case hardened gears

• Reduces gear life

• Reduces load-bearing capacity• Can lead to macropitting

• Can lead to wear

Micropitting Basics-Failure Modes

(Source: Germanischer Lloyd)



• Wear occurs on gear flanks

– Mostly in dedendum• High Sliding

– Increases in severity with load cycles

Micropitting Basics - Wear



Solution



Superfinishing

• Modifies topography

• Eliminate Micropitting

• Increases Lubricant Life and Cleanliness

• Increase Component Life• Easy to Implement



How It Works



Definition-ISF ®

ISF stands for Isotropic Superfinish

A Ground Surface has directionally

oriented, parallel rows of surface

asperities.

An Isotropic Superfinished surface

has no asperity peaks, only a non-

directional surface texture.



Superfinishing-ISF ®

• Chemically Accelerated Vibratory Finishing• Reduces Surface Roughness to < 0.1 µm Ra

• Planarizes and Micro-Textures the Surface

• Maintains Component’s Geometry

High Quality Vibratory Equipment High Density, Non-abrasive Media



Superfinishing Process



Surface Roughness Reduction



Planarization Mechanism



Why It Works



Quick Review of Lubrication Theory

• A low film thickness/surface roughness (λ) results in: – Boundary lubrication

– Mixed lubrication

• A high (λ) completely separates two load-bearingsurfaces and results in:: – Hydrodynamic lubrication



Quick Review of Lubrication Theory

(Source: KEW Engineering, Ltd)



• Suppose that two gears operating under theirmost severe conditions have a minimumlubricating film thickness of 100 µm.

• If it is experimentally determined that when thetwo gears are separated by a minimumlubricating oil film of 10 µm, the gears do notmicropit.

• Then the Safety Factor against Micropitting is100 µm / 10 µm = 10.

Micropitting Safety Factor for Gears



Sλ = λGF,min / λGFP

• Sλ is the safety factor against micropitting

• λGF,min is the minimum specific lubricant film thickness

• λGFP is the permissible specific lubricant film thickness

Micropitting Safety Factor in ISO TR15144



λGF,Y = hY / Ra

• λGF,Y is the local specific lubricant film thickness

• hY is the local lubricant film thickness. – This might be measured or calculated

– Not relevant to this presentation as only considering a reduction insurface roughness (R

a

)

• Ra is the effective arithmetic mean roughness value

Where

Ra

= 0.5(Ra1 +

Ra2

)

Ra1 = arithmetic mean roughness value of Driver

Ra2 = arithmetic mean roughness value of Driven

Local Specific Film Thickness



• It’s obvious then, that the Safety Factor can be increasedby:

– Increasing λGF,min , and/or

– Decreasingλ

GFP

• Since λGF,min is the minimum value of λGF,Y = hY / Ra

• Reducing Ra will increase λGF,Y.

Increasing the Safety Factor



For typical wind turbine gears were:

• Ra (Ground Surface) = 0.5 µm

• Ra (Superfinished Surface) = 0.1 µm

Then it is obvious that for the same gearbox

• λGF,min (Superfinished Surface) = 5 λGF,min (Ground Surface)

Conclusion:

• Superfinishing the gears to a 0.1 µm Ra increases the MicropittingSafety Factor by a factor of 5

Increasing λGF,min



1. KISSsoft, AG refers to a chart from the presentation Tribologische Aspekte bei Zahnradgetrieben - Speziell für Fahrzeuge – Prof.Dr.-Ing. Wilfried J. Bartz, T+S Tribologie und Schmierungstechnik,Denkendorf 5. Internationales CTI Symposium

2. They adapt it to wind turbine gears

3. Conclude that there are only three remaining practical methods ofimproving gears to prevent micropitting:

1. Superfinishing

2. Change the gears macro geometry

3. Change the gears micro geometry

Note: Once designed and in production, the only practical method isSuperfinishing.

Support for Previous Conclusion

S t f C l i B t Ch t A li d t WT b KISS ft AG



Action Effectiveness Comments

Material,

Surface

Reduce flank roughness to 1/16 1:2 See use of superfinishing gears in wind industry

Gears with run-in vs. without

run-in

1:3 Running in of gears may be done during serial testing of

gearboxes. The effectiveness may be lower for superfinished

gears and gears with micro geometry modifications.

Case carburizing vs. Nitriding

vs. Phosphating vs. Use of

copper plating

1:2:1.4:3 Seems to be impractical since only case carburized gears are

used for external gears in wind industry.

Normal vs. stainless steel 1:0.3 The use of stainless steel is limited.

Lubricant Use of EP Additives 1:5 The use of lubricants with EP additives and other additives is

considered state of the art in wind industry.

Double the viscosity 1:1.15 The viscosity of lubricants as used in the wind industry is

typically v40 = 320 mm2/s

Gear

Design

Change in gear macro

geometry

1:6 An optimal gear design is in the responsibility of the gear

designer.

Change in gear micro geometry 1:2 Suitable modifications of the gear geometry should be applied

by the gear designer.

Spur vs. helical gears 1:0.75 Typically, helical gears are used due to the high requirements in

terms of noise levels.

Operation High vs. low circumferential

speeds

1:8 Circumferential speed is determined by the rotor speed and gear

size and is hence difficult to change.

Green: relevant and may be applied by gear designer

Yellow: partly relevant as already state of the art in wind industry

Red: not applicable in wind industry

Support for Conclusions-Bartz Chart Applied to WT by KISSoft, AGTribologische Aspekte bei Zahnradgetrieben - Speziell für Fahrzeuge –

Prof. Dr.-Ing. Wilfried J. Bartz,

T+S Tribologie und Schmierungstechnik, Denkendorf 5. Internationales CTI Symposium



• KISSsoft, AG confirms Superfinishing to a 0.1 µm Ra

increases the Micropitting Safety Factor five times – Ground Flanks

• Rz: 4.5 µm

• Ra: 0.45 µm

– Superfinish Flanks

• Rz: 1.0 µm• Ra: 0.10 µm

Support for Conclusion-Increasing λGF,min (continued)

Un software per il calcolo a micropitting, Stefan Beermann e Ulrich Kissling, KISSsoft AG Hanspeter Dinner, EES KISSsoft

GmbH, ,Organi di trasmissione - settembre 2010



Micropitting Test Results

Th Eff t f S fi i hi G Mi itti



• Part I reported results from the FZG Brief Test of Gray

Staining (BTGS)

• Low micropitting resistance oil - FVA2 +LZ677A

• Standard FZG-C gearsets

• Baseline: Ra = 0.5 +/- 0.1 micron (ground)

• Superfinished: Ra = 0.1 micron

• Failure definition: 7.5 micron mean max profile deviation

• Testing Location: Technical University of Munich

The Effect of Superfinishing on Gear Micropitting

Part I- STLE 2008

Th Eff t f S fi i hi G Mi itti




Part I-STLE 2008

• Superfinished gears did not

fail.

•Superfinished Results-LS-9 – < 3.5 µm

• Baseline Results-LS9

– 9.7 and 10.3 µm



Part II – Test Specifics

• Micropitting test per FVA-Information-sheet 54/I-IV

• Lubricant – Mineral oil – ISO viscosity class 200 – Additive to reduce the

micropitting carryingcapacity

– Injected at 60 ºC


Part II-AGMA FTM 2008

• Baseline: Standard-FZG-C-gear set – 0.5 µm Ra

• Superfinished: Standard-FZG-C-gears – Nominal 0.1µm Ra

• Pitch line velocity: 8.3 m/s

• Test 1: Load Stage Test

• Test 2: Load Stage and Endurance

•Test Location: Ruhr University Bochum



Micropitting Resistance

FZG Micropitting Test (FZG Type C), FVA Sheet 54

Test Run 1

0

10

20

30

40

50

60

70

80

90

100

Initial 5 6 7 8 9 10

Load Stage

M i c r o

p i t t i n g A r e a o f G e a r F l a n k ( % )

Baseline

Superfinished

Test Run 2

0

20

40

60

80

100

Initial 5 6 7 8 9 10 8 10 10 10 10 10

Load Stage

M i c r o p i t t i n g

A r e a o f G e a r F l a n k ( % )

Baseline

Superfinished

Test Run 2 consisted of a completed Load

Stage Test followed by an Endurance Test.

The Endurance Test starts with an 80-hour

cycle at load stage 8, followed by five 80-

hour cycles at load stage 10

Test Run 1 was the Load Stage Test in

which the loading was increased every 16

hours starting with load stage 5 and ending

after load stage 10



Load Stage and Endurance Test

Micropitting = 79 %

Baseline Tooth Flanks

L d St d E d T t



Load Stage and Endurance Test

Thin gray mark isnot micropitting it

was due to a lack of

tip relief.

Superfinished tooth flanks

Micropitting = 0%



REM, The ISF Process, and the Wind Turbine

Market

A B i f Hi t f REM S fi i hi



A Brief History of REM Superfinishing

Technology developed in the early 1980’s for decorative finishes.

Engineered Surface Finishing started in the late 1980’s.

Isotropic Superfinish (ISF®) Processing of gears started in the early 1990’s.

ISF® Processing of large gears and extremely small gears started in the early

2000’s.

ISF® Process is applied to gear refurbishment in mid 2000’s.

AfterBefore

Hi t i Wi d E



History in Wind Energy

2003

• Winergy AG agrees to test ISF®

– Winergy receives exclusive utilization of ISF® in Wind

Energy Market on Gearboxes

2007• REM begins refurbishment of used gearboxes

2010

• Winergy and REM amend exclusive agreement

– REM provides ISF® Process to any Wind Turbine Gearbox

Hi t i Wi d E



History in Wind Energy

• New Gearboxes to date

– >9,000 MW class

• Refurbished Gearboxes

– >200 MW and KW class



Conclusions

C l i



• Superfinishing reduces Ra and modifies surfacetopography to a planarized condition

• Superfinishing eliminates micropitting

• Superfinishing wind turbine gears to 0.1 µm Raincreases the Micropitting Safety Factor by five – The following calculations support the claim:

• ISO TR 15144-1

• KISSsoft, AG papers

Conclusions

C l i



• FZG-C gears superfinished to 0.1 µm Ra

– Did not micropit during a FZG Brief Test of Gray

Staining (BTGS) using Low micropitting resistance oil

– Did not micropit during 2 independent FZG load stagetests at LS10

– Did not micopit during an endurance test consisting of 580 hr tests at LS10.

Conclusions

R f



• Morphology of Micropitting, R.L.Errichello, AGMA FTM, 2011, 11FTM17• Tribologische Aspekte bei Zahnradgetrieben - Speziell für Fahrzeuge,Prof. Dr.-

Ing. Wilfried J. Bartz, T+S Tribologie und Schmierungstechnik, Denkendorf 5.

Internationales CTI Symposium

• Un software per il calcolo a micropitting, Stefan Beermann e Ulrich Kissling,

KISSsoft AG Hanspeter Dinner, EES KISSsoft GmbH, ,Organi di trasmissione -

settembre 2010

• The Effect of Superfinishing on Gear Micropitting (Part I), L. Winkelmann, P. B.

King, M. Bell, O. Elsaeed, STLE Annual Meeting, May, 2008.

• The Effect of Superfinishing on Gear Micropitting (Part II), ), L. Winkelmann, M.

Bell, O. Elsaeed, AGMA FTM, 2008, 08FTM10.

References

Additional Questions?



Additional Questions?

www.remchem.com

© 2011 REM Chemicals, Inc. All rights reserved REM, ISF and REM Surface Engineering are registered

trademarks of REM Chemicals, Inc. This presentation contains confidential and proprietary information of REM

http://www.remchem.com/

http://www.remchem.com/

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Surface Roughness and Micropitting -day1_sessioni_4_rem_winkelmann.pdf